1. Field of the Invention
The present invention relates generally to material forming; and more particularly, to a method for forming a metal sheet.
2. Description of Related Art
Various methods are known for forming a metal sheet. One method involves a draw process wherein a punch pulls a portion of the metal sheet through a shaped die. During the process, the metal sheet typically undergoes a reduction or change in the cross-sectional area or wall thickness of the sheet. Such processes are typically limited by the material's ability to be strained past its rupture point. Thus, depending upon the complexity of the part, the forming stresses on the metal sheet during the forming process may result in metal failure or fatigue and correspondingly an unusable or scrap part.
Superplastic forming (SPF) is a process that takes advantage of a material's superplasticity or ability to be strained past its rupture point under certain elevated temperature conditions. Superplasticity in metals is defined by very high tensile elongation and is the ability of certain materials to undergo extreme elongation at proper temperature and strain rate. SPF is a process used to produce parts that are difficult to form using conventional fabrication techniques.
During the superplastic forming process, the metal sheet, or as often referred to the blank, is heated to a point of superplasticity after which a predefined gas pressure is applied to one side of the sheet. The pressure forces the sheet into a die cavity while maintaining a target strain rate for deforming the sheet throughout the forming cycle. The superplasticity of the material enables forming of complex components that normally cannot be formed by conventional room temperature metal forming processes. Use of a superplastic forming process enables forming a workpiece with a deep cavity or one formed over very small radii. Superplastic forming does have a disadvantage in that it normally requires relatively long forming cycle times. Specifically, a conventional SPF process used to manufacture a complex part can require a forming cycle time as high as 30 minutes.
Further, superplastic forming cannot always be used to obtain a complex part in a single step and therefore may require two or more forming steps. U.S. Pat. No. 6,581,428 illustrates one method for forming a part that uses a single die capable of preforming both a mechanical draw process and superplastic forming process. The '428 patent utilizes a pre-forming punch disposed on one of the die members, wherein the punch pre-forms the blank prior to an application of gas pressure to the blank to complete the forming process. While this die structure and corresponding process is well suited to many applications, the die structure is somewhat complex and may not accommodate forming some aspects of a complex part such as small radii and corners without causing wrinkling during the drawing process.
In addition, such die sets can be somewhat expensive, relatively complex and in some instances, suitable material from the binder area may not be pulled into the die during the draw process. Accordingly, there exists a need for a process for forming metal sheets or blanks that reduces the complexity of the die components while making use of both mechanical and superplastic forming processes.